1. Field of the Invention
The present invention relates to a finder optical system, and more particularly to a finder optical system which utilizes light flux passing through a photo-taking lens (objective lens) and is applied to a video still camera or the like.
2. Description of the Prior Art
In comparison with a single lens reflex camera of 35 mm format, the video still camera using a solid state image device such as a CCD (charge coupled device) having a small image plane necessitates an eyepiece of short focal length capable of obtaining a high finder magnification. Accordingly, where a finder used for the single lens reflex camera is used for the video still camera, it is required to shorten the optical path length in an erecting optical system after the focal surface of the objective lens.
Also, in the video still camera, due to apparatuses such as an image signal processing circuit, the rear surface of the camera body protrudes greatly toward a pupil of the eye in comparison with the single lens reflex camera. Therefore in the video still camera having the eyepiece unit of a total length nearly equal to that of the conventional single lens reflex camera, the pupil of the eye cannot be brought close to the eyepiece and the light flux relating to the peripheral part of the finder image is eclipsed.
Furthermore, the pentagonal roof prism used for the single lens reflex camera leads to a small finder system in the direction of the objective lens in a manner that the light flux is reflected once toward the object side when an inverted image formed on the focal surface is restored to an erect image. Therefore where the pentagonal roof prism is used in the video still camera, it becomes increasingly difficult to reduce the distance between the eyepiece unit and the pupil of the eye.
Consequently, three principal conditions required for the erecting system of the finder optical system applied to the video still camera are as follows:
(1) The optical path length in the erecting system has to be shortened.
(2) The erecting system has to be brought close to the pupil of the eye.
(3) Up-down inversion and right-left inversion of the light flux of odd-number times are respectively required.
For the above-mentioned requirement (2), it could be considered to accommodate this requirement by elongating the total length of an eyepiece unit, but this is not so preferable because the magnification of the finder tends to be reduced as the total length of the eyepiece unit is elongated.
Now, considerations are made on the erecting system which fulfills the above-mentioned three requirements. First, from the viewpoint of shortening the optical path length and simplifying the constitution, up-down inversion and right-left inversion are performed once respectively. For this purpose, a constitution is considered which provides a roof surface performing right-left inversion and one reflecting flat surface facing this roof surface and performing up-down inversion. Furthermore, it is considered that to bring the erecting system close to the pupil of the eye, the light flux is reflected to the exit pupil side when right-left inversion is made. For such an erecting system, the system as disclosed in FIG. 1 of each of the Japanese Laid-Open Utility Patent Publications No. 163941/1976 and No. 182726/1982 (FIG. 1 of the present application) has been known, wherein the erecting system is constituted with a prism.
However, the above-mentioned erecting system presupposes to use the light flux which is incident on the objective lens and reflected in the vertical direction, and therefore this system is not yet enough to fulfill the above-mentioned requirements (1) and (2). In addition, it is very difficult to make the light flux projected from the erecting system horizontal i.e. parallel to the light flux incident on the objective lens, and if it is intended to be projected from the erecting system at an angle close to horizontal direction (direction of the objective lens), the optical path length in the erecting system becomes very long, and it becomes more and more difficult to fulfill the requirement (1).